PROJECT SUMMARY/ABSTRACT Mentalizing, or the ability to attribute mental states to the self and others and to understand how these affect behavior, forms the basis of humans' capacity to build and maintain relationships with one another. As such, it is a key construct of RDoC's Systems for Social Processes. Mentalizing deficits can be categorized as hypermentalizing, hypomentalizing, or no mentalizing and are found in a range of pathology, contributing to poor psychosocial functioning. Adolescence is a key developmental period in the study of mentalizing given dramatic changes in the adolescent social brain that coincide with social reorientation. Adolescence is also characterized by peak onset for psychiatric disorders, and mentalizing deficits have been shown to explain the interpersonal problems associated with these disorders. Despite the importance of mentalizing for adolescent mental health, existing tools assess mentalizing as a stagnant property of a single individual, relying mostly on non-self-referential stimuli or hypothetical scenarios. Further, existing tools fail to take into account the developmental origins of mentalizing, which are based in the parent-child relationship. This research proposal addresses this gap by developing (Aim 1) and evaluating (Aim 2) a tool that can measure mentalizing as it occurs during a real-time parent-adolescent interaction. To address Aim 1, we will use archival data of parent-adolescent dyads participating in a conflict-discussion paradigm and follow Bakeman and Gottman's (1997) approach to observational coding system development to develop a coding manual. With archived videos, reliability and face validity will be evaluated. For Aim 2, we will test the validity of the coding system against existing measures of mentalizing, relationship quality, and borderline personality disorder, which emerges in adolescence and has been shown to be a test case of psychopathology where mentalizing goes awry. By creating the first observational measure of real-time mentalizing based in the parent-adolescent relationship, the proposed research promises to operationalize an important RDoC construct for use by other scientists and clinicians. This is in line with the National Institute of Mental Health's (NIMH) mission to transform the understanding of mental illness by identifying behavioral indicators with high predictive value across disorders. Carrying out the fellowship research will increase our understanding of mentalizing, a process that supports healthy development across the lifespan with the introduction of a more ecologically valid assessment. In conclusion, the award and completion of this project will not only support substantial training in research methods and dissemination to a promising young researcher, but will address a significant need for improved assessment of adolescent mentalizing to inform future research and intervention.